The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of methanol to hydrocarbons and especially light olefins (by light olefins is meant C.sub.2 to C.sub.4 olefins). The interest in the methanol to olefin (MTO) process is based on the fact that methanol can be obtained from coal or natural gas by the production of synthesis gas which is then processed to produce methanol.
Processes for converting methanol to light olefins are well known in the art. Initially aluminosilicates or zeolites were used as the catalysts necessary to carry out the conversion. For example, see U.S. Pat. Nos. 4,238,631; 4,328,384, 4,423,274. These patents further disclose the deposition of coke onto the zeolites in order to increase selectivity to light olefins and minimize the formation of C.sub.5 + byproducts. The effect of the coke is to reduce the pore diameter of the zeolite.
The prior art also discloses that silico aluminophosphates (SAPOs) can be used to catalyze the methanol to olefin process. Thus, U.S. Pat. No. 4,499,327 discloses that many of the SAPO family of molecular sieves can be used to convert methanol to olefins. The '327 patent also discloses that preferred SAPOs are those that have pores large enough to adsorb xenon (kinetic diameter of 4.0 .ANG.) but small enough to exclude isobutane (kinetic diameter of 5.0 .ANG.). A particularly preferred SAPO is SAPO-34.
U.S. Pat. No. 4,752,651 discloses the use of nonzeolitic molecular sieves (NZMS) including ELAPOs and MeAPO molecular sieves to catalyze the methanol to olefin reaction.
The effect of the particle size of the molecular sieve on activity has also been documented in U.S. Pat. No. 5,126,308. In the '308 patent it is disclosed that molecular sieves in which 50% of the molecular sieve particles have a particle size less than 1.0 .mu.m and no more than 10% of the particles have a particle size greater than 2.0 .mu.m have increased activity and/or durability. The '308 patent also discloses that restricting the silicon content to about 0.005 to about 0.05 mole fraction also improves catalytic performance.
In contrast to this art, applicants have found that molecular sieves having the empirical formula (EL.sub.x Al.sub.y P.sub.z)O.sub.2 (hereinafter ELAPO) where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof and x, y and z are the mole fractions of EL, Al and P respectively and having a predominantly plate crystal morphology wherein the average smallest crystal dimension is at least 0.1 micron and has an aspect ratio of less than or equal to 5. These molecular sieves produce a higher amount of ethylene versus propylene. This increased selectivity is a very desirable feature of a MTO catalyst. This morphology is obtained by controlling the metal (EL) content of the molecular sieve and the crystallization time during synthesis of the molecular sieve.